Significant developments in mobile communication technology and the extension of mobile communication networks have resulted in the development of a vast array of additional services and applications for use with a cellular phone. Concurrently, demand among cellular phone users for additional services such as a location service, a multimedia service and a push-to-talk (PTT) service is increasing. The PTT service supports various supplementary functions such as instant messenger, a status display and a group call and a voice call which are also provided by an existing radio or a trunked radio system (TRS).
Standardization of a push-to-talk-over-cellular (PoC) service that employs the PTT function in a mobile communication network is currently taking place. One unique feature of the PoC service is that a user can participate in a plurality of PoC sessions and so can switch from the PoC sessions to use a call service. This feature is a requirement that is specified in the open mobile alliance (OMA), which is a forum for specifying mobile communications services.
FIG. 1 is a schematic diagram of a conventional PoC service system. Referring to FIG. 1, a PoC client 10, which is a service requester installed in a mobile station, i.e., a PoC terminal, is connected to a SIP/IP core network 30 which supports session initiation protocol (SIP) and internet protocol (IP) multimedia functions via an access network 20.
The PoC client 10 resides in the PoC terminal to provide access to the PoC service. The main function of the PoC client 10 is to initiate a PoC session, participate in a PoC session that is currently proceeding and terminate a PoC session. The PoC client 10 makes and transfers a talk burst, supports an instant personal alert and performs authentication when accessing the PoC service. Hereinafter, unless otherwise stated, the PoC client 10 is assumed to be the same as a PTT service subscriber.
The SIP/IP-based core network 30 is connected to a PoC server 60, a group list management system (GLMS) 50 and a presence server 70 in order to support the PoC service.
The PoC server 60 has a controlling PoC function for maintaining a PoC session, or a participating PoC function for participating in a PoC session for a one-to-one PoC call or a one-to-two or more PoC call (or group PoC call).
Functional blocks of the PoC server 60 will be explained below with reference to FIG. 2.
FIG. 2 is a schematic diagram of a conventional PoC server.
A function of the PoC server is classified into a controlling PoC function (CF) for maintaining a PoC session and a participating PoC function (PF) for maintaining each PoC session, which will be explained below with reference to relevant tables.
TABLE 1Controlling PoC Function (CF)Provides centralized PoC session handlingProvides centralized Media distributionProvides centralized Talk Burst Arbitration functionality including talkeridentificationProvides SIP session handling, such as SIP session origination, termina-tion, etc.Provides policy enforcement for participation in group sessionsProvides participant informationCollects and provides centralized media quality informationProvides centralized charging reportsMay provide transcoding between different codecsSupports Talk Burst Control Protocol Negotiation
As described in Table 1, the CF maintains a PoC session. The PoC server receives requests for the floor from PoC clients, arranges an order in which the clients are given the floor and gives the clients the floor in that order. The PoC server also distributes a talk burst from a specific PoC client to all PoC clients participating in a group PoC call, and provides information regarding the PoC clients participating in the group PoC call.
As described below in Table 2, the PF manages a PoC session between the CF and each PoC client. The PF relays the floor to the PoC client from the CF. The PF relays media between the CF and the PoC client, provides transcoding between different codecs and provides a filtering function for filtering one of two PoC sessions chosen by a user when there is simultaneous talking in two active PoC sessions.
TABLE 2Table 2 Participating PoC Function (PF)Provides PoC session handlingMay provide the Media relay function between PoC Client and ControllingPoC serverMay provide user media adaptation proceduresMay provide the Talk Burst control message relay function between PoCClient and Controlling PoC serverProvides SIP session handling, such as SIP session origination, termina-tion, etc, on behalf of the represented PoC ClientProvides policy enforcement for incoming PoC session (e.g. access con-trol, incoming PoC session barring, availability status, etc.)May collect and provide media quality informationProvides the participant charging reportsMay provide filtering of the media streams in the case of simultaneoussessionsMay provide transcoding between different codecsMay support Talk Burst Control Protocol NegotiationStores the current Answer Mode and Incoming PoC Session Barring pre-ferences of the PoC Client
The PoC service system comprises the GLMS 50, which provides a PoC group and a PoC group list, which are necessary for setting up a PoC group session, and a group management/administration unit 40 that manages the PoC groups and the list.
The PoC service system further comprises the presence server 70, which provides and maintains information as to whether or not there is an individual or a group that desires the PoC service, and a remote PoC network 80 which comprises, for example, a SIP/IP core network and a PoC server of another user to provide a connection with another user for a one-to-one PoC call or a one-to-many PoC call.
In the above PoC service system, a PoC user can input information about a group and group members to the GLMS 50 through his/her PoC terminal, and can receive information about PoC users whom he or she can call through an individual or group list transmitted from the GLMS 50. Alternatively, information about a group and group members in the GLMS 50 can be input, corrected and managed via a reliable communication network such as the Internet or Intranet.
The PoC user registers a PoC address of his/her PoC terminal in the SIP/IP core network in order to use the PoC service. Upon a request of the PoC user, the SIP/IP core network stores PoC user information. Thus, when another PoC user tries to request a group PoC call, the PoC user registers his/her information in the SIP/IP core network in advance and requests a PoC call to his/her SIP/IP core network using group identification information transmitted from the GLMS 50. The SIP/IP core network performs address determination and domain location determination using information of the call requesting PoC user, and then transfers a PoC call request to a home PoC server with which the call requesting PoC user is registered. The PoC server prepares for establishment of a PoC session, obtains each user's information from the GLMS 50 at the PoC call request, and then transfers a PoC call request signal to a corresponding SIP/IP core network. When a PoC call request is made to users within an Intradomain, the PoC server performs both the CF and the PF. The PoC server that manages a call-requested PoC user requests a PoC call to the PoC user using information it receives from the PoC user after the SIP/IP core network performs the location determination procedure.
FIG. 3 is a flowchart illustrating a conventional PoC session call set-up procedure.
A PoC client A transmits to a SIP/IP core network A an INVITE message containing information about an individual or a group to which it desires to talk. The SIP/IP core network A recognizes a parameter that represents the PoC service from information of the INVITE message sent from the PoC client A and transmits an INVITE request to a PoC server A that manages the PoC client A. A function of the PoC server A as a home PoC server is implemented depending on a PoC call type. For example, when it sets up a PoC session such as a one-to-one PoC call or an ad-hoc group PoC call, its home PoC server function is implemented to perform both the CF and PF and transmits the INVITE message to a PoC server B that manages a PoC client B. In the case of a pre-arranged group PoC call, a session management function cannot be implemented in the home PoC server since it participates in a previously arranged PoC session. Thus, the INVITE message is transmitted to the PoC server A which performs the CF. The SIP/IP core network serves as a passage for relaying the INVITE message and searching for an address of the PoC server A or B. When the INVITE message is transmitted to the PoC client B, the PoC client B transmits a response to the PoC client A such as alerting, ringing, a light or a beep. When the PoC client B accepts the PoC call, an OK response is transmitted to the PoC client A. As a result, a PoC call is set up. That is, the PoC call is finally established when the PoC client A takes the floor from the PoC server that performs the CF.
The PoC service described above provides a PoC call between two individuals or a group PoC call between more than two individuals, and also supports a multiple PoC session in which one PoC client participates in a plurality of group PoC calls using a PoC terminal. During the group PoC call, some members of one group may desire to divide a PoC group session. The PoC group session can be divided when there are too many members participating in one PoC group session to take the floor or some members of the group desire to perform a separate group PoC call.
In the conventional PoC service, the PoC user can participate in only one PoC group session or in a plurality of PoC sessions. When some members of the group desire to divide a new group PoC session from one PoC group session to perform a PoC call in the new PoC group session that includes specific members, the members terminate the existing PoC group session and then set up the new PoC group session.
FIG. 4 is a flowchart illustrating a method for dividing a PoC group session according to the conventional art.
In FIG. 4, a PoC session X is a group PoC session to be divided at some members' request, and a PoC client X and a PoC client Y perform a PoC group session procedure for receiving a talk burst from a PoC server X to take the floor. In order for the PoC client Y to divide the currently proceeding PoC group session, a session termination request “BYE Request” is transmitted to the CF that manages the currently proceeding PoC group session. As the session termination procedure, media parameters of the PoC session X that are set between a server and a client are released. After terminating the PoC session X, the PoC client Y transmits an INVITE request to set up a new PoC session. The PoC client Y transmits the INVITE request to a PoC server of a home network so that the PoC server can become a CF for managing the new PoC session. Other PoC users who have already terminated the existing PoC group session are then requested to set up the new PoC session to perform an independent talk burst control procedure.
One benefit of the PoC call is low cost. There may be a situation in which some members participating in a group PoC call desire to divide a group PoC session to open a new PoC session for a group PoC call in which only specified members participate. However, the conventional PoC service should undergo a procedure for terminating the currently proceeding PoC group session to divide the PoC group session. This can cause a problem wherein the PoC users have to wait during a termination operation. Furthermore, an initializing procedure, for example, for setting media parameters should be performed again when the group members are to be re-connected.